1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, particularly to a configuration for correcting skew feeding of a sheet such as recording paper to an image forming portion.
2. Description of the Related Art
Conventionally, the image forming apparatus such as a copying machine a printer, and a facsimile includes the sheet conveying apparatus which conveys the sheet such as the recording paper in the image forming portion. Some examples of sheet conveying apparatus include skew feeding correction portions which correct the sheet skew feeding to align an attitude and a position of the sheet until the sheet is conveyed to the image forming portion.
In such skew feeding correction portions, a loop is formed in the sheet with a pair of registration rollers to correct the skew feeding. However, because the sheet is temporarily stopped, a time necessary to correct the skew feeding becomes lengthened.
Therefore, in order to shorten the time necessary to correct the skew feeding, there is an active registration method in which the sheet is rotated to correct the skew feeding while conveying the sheet using two sensors and two pairs of skew feeding correction rollers independently rotated (see, for example, Japanese Patent Publication Laid-Open No. 10-032682).
In the active registration method, the skew feeding is detected at a front end of the sheet based on a sheet detecting signals from the two sensors when the front end of the sheet transverses the sensors provided on a coaxial line orthogonal to a sheet conveying direction in a sheet conveying path.
Then, a sheet skew feeding amount is detected based on the sheet detecting signals from the two sensors. Then, rotating speeds of two drive motors for driving two pairs of skew feeding correction rollers are controlled according to the detected skew feeding amount, whereby the sheet conveying speeds of the two pairs of skew feeding correction rollers are changed to correct the sheet skew feeding according to the sheet skew feeding amount.
During the skew feeding correction, the sheet conveying speed of one of the pairs of skew feeding correction rollers is reduced (referred to as skew feeding speed reducing control) or increased (referred to as skew feeding speed-increasing control) with respect to the other pair of skew feeding correction rollers according to the sheet skew feeding amount, thereby correcting the sheet skew feeding.
In the active registration method, because the skew feeding is corrected without tentatively interrupting the sheet conveyance, a sheet interval (interval between a precedence sheet and a following sheet) can be narrowed compared with other methods. Therefore, sheet conveying efficiency can be enhanced, and an overall image forming speed can substantially be improved without increasing an image forming process speed in the image forming apparatus. Recently, the image forming process speed has tended to increase and, accordingly, the active registration method can provide speed enhancements of the sheet conveyance process to match such speed enhancements of the image forming operation in the image forming apparatus.
In the conventional image forming apparatus including the skew feeding correction portion having the above configuration, it is necessary to correct a position in the sheet conveying direction in addition to the sheet skew feeding correction.
Therefore, for example, the conventional image forming apparatus includes a correction roller which is located on a downstream side of the skew feeding correction roller to correct the position in the sheet conveying direction. After the skew feeding is corrected by the skew feeding correction roller, the rotating speed of the correction roller is controlled to change the sheet conveying speed such that the sheet is conveyed at ideal timing at which the front end of a toner image is aligned with the front end of the sheet.
However, in the case where the sheet conveying speed of the skew feeding correction roller is controlled for the skew feeding correction, the position of the sheet fluctuates in the sheet conveying direction depending on the decrease in speed on the sheet preceding side or the increase in speed on the sheet following side.
For example, the sheet conveyance tends to be delayed (lagging) in the case of the skew feeding speed-reducing control. Therefore, sheet conveying lag is increased when the sheet conveyance is lagging compared to a skew feeding correction start position. As used herein, the sheet conveying lag shall mean that the sheet conveyance is lagging compared with the timing of the ideal sheet conveyance.
The sheet conveyance tends to be advanced (leading) in the case of the skew feeding speed-increasing control. Therefore, sheet conveying lead is increased when the sheet conveyance is leading compared to the skew feeding correction start position. As used herein, the sheet conveying lead shall mean that the sheet conveyance is leading compared with the timing of the ideal sheet conveyance.
That is, when skew feeding correction is performed by the skew feeding correction roller, the sheets after skew feeding correction may have a lag amount or lead amount which should be corrected in a correction roller located on the downstream side of the skew feeding correction rollers. The lag amount may be especially serious when the speed-reducing correction is performed on a sheet which reached the skew feeding correction rollers in the sheet conveying lag state. Similarly, the lead amount may be especially serious when the speed-increasing correction is performed on a sheet which reached the skew feeding correction rollers in the sheet conveying lead state. In such cases, a sheet conveying speed of the downstream correction roller may be increased or decreased temporarily (with respect to a normal or target speed) to correct for the lag amount or the lead amount of the sheet after skew feeding correction. In particular, a speed-increasing period or a speed-reducing period of the correction roller is increased to lengthen the time for which the sheet conveying speed of the downstream correction roller is increased or decreased with respect to the target speed during the correction. However, because a probability of generating slip of the correction roller is increased during the speed-increasing period or speed-reducing period, accuracy of positional correction may in practice be decreased in the sheet conveying direction.
As shown in FIG. 15, in the actual speed control of the downstream correction roller, the speed is changed in a stepwise manner, and the correction time is limited to integer multiples of a period of the target speed V1. Therefore, an error is generated with respect to an ideal analog waveform, and an amount of error is increased as the speed-increasing period or speed-reducing period is broadened, whereby the correction accuracy is decreased.